Most animals are targets of the predatory efforts of other animals. Behavioral self-defense takes may forms, most notably vigilance, crypsis, and active defense. Body size, may enhance behavioral protections from predation. Finally, reproductive adaptations, such as predator saturation, have important behavioral consequences.


Vigilance, watching, smelling and listening for predators, is one of an animal's most important protections. An animal which fails to be vigilant likely ends up being a meal for a predator. Vigilance raises two important issues:

  • An animal which is vigilant all the time does not perform other necessary functions, such as feeding. Consequently vigilance becomes part of an animal's time budget and a strategy for maximizing the benefits of vigilance, while at the same time managing other time demands, must evolve.
  • Vigilance can be shared. Animals living in social groups can rely on other animals to be vigilant part of the time. Shared vigilance may be a driving force in causing animals to group together, such as schooling fish, herding ungulates, or flocking birds. Hypotheses for why animals group together include the selfish herd, reciprocal altruism, and kin selection.


An alternative to perpetual vigilance is being well-hidden. We think most often about morphological crypsis--animals which blend into their background. Behavioral crypsis is important as well. This includes unflinching inactivity in the close presence of a predator, subtle and stealthy movements, activity cycles which evade the predators' prime hunting times, and behavior which mimics other, less desirable, prey items.

Active defense

Many animals have weapons which can be employed in defense against predators.Sometimes these weapons have evolved as anti-predator defenses, such as honey bee stings, but often these weapons evolved for use in predation or territorial and mating conflict, and are co-opted for use in self-defense. Sharp teeth, hard hooves, and horns and antlers can all be used in self-defense.

Poisons and chemical feeding deterrents carried in body tissues are also a form of active defense. These are common insects, such as the monarch butterfly and in marine invertebrates. A few vertebrates, such as poison-arrow frogs and birds are poisonous as well. Mammals do not produce poisonous body tissues, but some mammals, such as the skunk, are well defended by chemical feeding deterrents. Venemous snakes use their venom defensively, and may couple their poisonous defense with warning coloration, as in coral snakes and eyelash vipers.

In some cases the best active defense is to flee. In many cases quick reflexes trigger rapid running, jumping, or flying responses to predators. A somewhat different response is seen in many insects, which simply drop to the ground (and dissappear in the leaf litter) when a bird or other predator appears.

Body size and predator saturation

Large body size is a major deterrent to predation. While size alone is not, strictly speaking, a behavior, size has impacts on many other aspects of an animal's biology. Elephants, hippopotami, and, some species of whale are good examples of species in which large size is a clear deterrent to predators. These large species are linked by their herbivorous or grazing feeding habits; they have little in the way of armaments to prevent predation other than their size.

Sometimes being the right size is more important than being a maximum size. In ** ants, workers block their nest entrances with their heads when a predator approaches. The precision of the match between head size and nest entrance size greater facilitates the anti-predatory behavior.

A final way to deal with predators is to present little or no behavioral defense. Instead, so many animals are produced that there are not enough predators to eat them all; this is called predator saturation. Examples of predator saturation include cicadas, mayflies, and **. In these cases natural selection favors a huge explosion of reproduction. The interesting outcome is that individual animals put up little or no defense against predation. It may surprise you that such easy pickings exist, but species which practice predator saturation survive because some individuals always escape predation. Predator saturation is usually coupled with reproductive behavior that is synchronized, so that the vulnerable animals all emerge at the same time, and with emergences that are far apart in time, so that predators cannot depend on a steady diet.

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copyright ©2001 Michael D. Breed, all rights reserved